The present invention relates generally to an automated system for inspecting and controlling fillet formation, and more specifically to a system and method for inspection of fillet formation along an edge(s) of a packaged microelectronic device, which is attached to a supporting substrate.
In the semiconductor industry, fillet may be found along one or more edges, including corners, of a packaged microelectronic device, e.g., a ball grid array (BGA), chip scale package (CSP), or flip chip, which is attached to a supporting substrate, e.g., a printed circuit board (PCB), via known dispensing processes. The fillet is an attribute of an underfill material that is incorporated into the assembly of the packaged microelectronic device and supporting substrate to add strength to the mechanical connections and to protect against environmental damage.
In one example, a defined amount of a curable underfill material, such as an epoxy, is dispensed from a dispensing device along one or more edges of a rectangular-shaped packaged microelectronic device, which has been previously soldered with solder bump interconnections or another type of attachment to a PCB. By capillary action, the material is drawn into the gap underneath the packaged microelectronic device and, as it flows outward to the other edges of the packaged microelectronic device, underfills the packaged microelectronic device, i.e., fills the gap between the packaged microelectronic device and the PCB. The fillet is formed around the packaged microelectronic device, which extends from the sides of the packaged microelectronic device to the PCB. In other words, the fillet is not under the packaged microelectronic device but forms along the edges of the packaged microelectronic device. The underfill material is eventually cured, typically thermally cured by heating, after fillet formation. The fillet may be uniform, or not, and may be further enhanced by a secondary dispensing process known in the art as a “seal pass”.
In another example, the dispensing process utilizes edge or corner bond dispensing of curable underfill material to form fillets known in the art, respectively, as edge bonds or corner bonds. In this process, the fillet is directly formed along one or more, or a portion of, the edges, including the corners, of the packaged microelectronic device, without underfilling the space between the packaged microelectronic device and supporting substrate. With corner bond dispensing, the dispensed material may only partially flow under the packaged microelectronic device to provide bonding. Again, the underfill material is eventually cured after fillet formation.
A fluxing or “no-flow” underfill process is yet another technique for forming fillets. In this process, underfill material is first dispensed on a supporting substrate's solder pads, then a packaged microelectronic device is placed on top of the underfill material. As the packaged microelectronic device is forced down onto the corresponding solder pads, the packaged microelectronic device displaces the undefill material. Excess material forms a fillet along the edges of the packaged microelectronic device. This assembly is then put through an oven that reflows the solder to attach the packaged microelectronic device to the supporting substrate and cure the underfill material at the same time.
The resulting assembly may be subjected to shock, vibration, thermal cycling, or other environmental stresses in its intended use. The underfill material, which includes the corner bond material, in each of the above processes helps improve the reliability and operational longevity of the resulting assembly.
Numerous variables that can affect fillet formation. The variables can include, for example, the viscosity, surface tension, volume, and/or temperature of the underfill material, as well as the surface characteristics and temperature of the packaged microelectronic device and supporting substrate. Those variables can be inter-dependent, e.g., temperature affects viscosity, and/or dynamic, i.e., change over time. Because precise control of the variables can be difficult to obtain, quality and consistency of the underfill dispensing process, likewise, can be difficult to achieve, as well as sustain once so achieved.
Conventional methods of monitoring fillet formation involve human inspection of the resulting fillet. For example, in one instance, the human inspector simply observes the size and shape of the fillet formed along the edge(s) of the packaged microelectronic device that is attached to the supporting substrate to determine whether the fillet is properly dimensioned. If the fillet is improperly dimensioned, the operating parameters of the underfill dispensing process, e.g., the temperature of the supporting substrate or amount of the underfill material, can be adjusted accordingly to change the fillet size and/or shape. Unfortunately, manual inspection exhibits numerous limitations. For example, the subjectivity of evaluating whether the fillet is properly dimensioned varies from operator to operator. In addition, traceability make be lacking for the manual inspection process.
It would thus be beneficial to provide an improved system and method for inspecting and controlling fillet formation that overcomes the aforementioned drawbacks.